Key Engineering Materials Vols. 656-657

Abstract: The effect of synthetic natural gas (SNG) and mixture of syngas and SNG fed to Natural Gas Combined-Cycle (NGCC) plants is presented in this study via a system-level simulation model. The commercial chemical process simulator, Pro/II^® V8.1.1, was used in the study to build the analysis model. The NGCC plant consists of gas turbine (GT), heat recovery steam generator (HRSG) and steam turbine (ST). The study envisages two analyses as the basic and feasibility cases. The former is the benchmark case which is verified by the reference data with the GE 7FB gas turbine. According to vendor’s specification, the typical net plant efficiency of GE 7FB NGCC with two gas turbines to one steam turbine is 57.5% (LHV), and the efficiency is the benchmark in the simulation model built in the study. The latter introduces a feasibility study with actual parameters in Taiwan. The SNG-fed GE 7FB based combined-cycle is evaluated, and the mixture of SNG and syngas is also evaluated to compare the difference of overall performance between the two cases. The maximum ratio of syngas to SNG is 0.14 due to the constraint for keeping the composition of methane at a value of 80 mol%, to meet the minimum requirement of NG in Taiwan. The results show that the efficiency in either case of SNG or mixture of SNG and syngas is slightly lower than the counterpart in the benchmark one. Because the price of natural gas is much higher than that of coal, it results in higher idle capacity of NGCC. The advantage of adopting SNG in Taiwan is that it could increase the capacity factor of combined-cycles in Taiwan. The study shows a possible way to use coal and reduce the CO₂ emission, since coal provides nearly half of the electricity generation in Taiwan in recent years.

Abstract: Performance testing for a single-cell solid oxide fuel cell (SOFC) stack is carried out to optimize its operating conditions. In this study, the Taguchi method is employed to effectively define the test matrix. The single-cell stack is composed of a 10x10 cm² commercial anode-supported cell, metallic interconnects, current collectors, and glass-ceramic sealant. The major parameters for the experiments include: flow rates of fuel (hydrogen) and oxidant (air) gases, and temperatures. The target indices are the electrical power output, electrical efficiency, and fuel utilization. The fuel flow rates (400, 500, and 600 sccm), air flow rates (1000, 1500, and 2000 sccm) and temperatures (650, 675, and 700°C) are evaluated for different experimental combinations. The results reveal that, the operating temperature is the most crucial factor to the stack performance. The maximum power reaches to 46 W (570 mW/cm²) with a current of 58 A (715 mA/cm²) at test conditions of 700°C and fuel and oxidant flow rates of 600 sccm and 2000 sccm, respectively. As the fuel flow rate decreases to 400 sccm, the electrical efficiency can reach to 53% while the power at 34.6 W (427 mW/cm²) and current 42 A (518 mA/cm²). As the current increases to 44 A (543 mA/cm²), the fuel utilization reaches to 83%, nevertheless concentration polarization is observed in such operating condition.

Abstract: The anode-supported solid oxide fuel cell (SOFC) with low-porosity anode structure is fabricated and the electrochemical characteristics are investigated. The electrochemical characterization of the cell shows a periodic oscillation phenomenon of the cell voltage under the constant current density operation. The low-porosity anode structure results in the decrease in the effective diffusion coefficient and the accumulation of water vapor. The cell voltage oscillation is mainly caused by the concentration polarization as well as the boundary migration of the reaction zone. The profound influence on the concentration polarization can be observed when the cell test is executed with operation condition of higher current density, lower hydrogen concentration, and lower hydrogen flow rate in the anode side.

Abstract: The dynamic properties of liquids in confined geometries or porous media are of both fundamental and practical importance in many physical situations, such as lubrication of micro/nanoelectromechanical systems, the flow of liquids in rocks and nanopores, and transport through porous media in filtration processes. The investigation of liquids confined at the nanoscale has been an active field for many years, but their properties remain controversial. In this work, a surface force apparatus (SFA) has been used to investigate the dynamic properties of nanoconfined octamethylcyclotetrasiloxane (OMCTS) between two mica surfaces. The dependences of normal and adhesion forces on different confinement or retraction rates were studied. The hydrodynamic effects and liquid drainage were also determined. The contribution of hydrodynamic effects to liquid drainage is limited. Our experimental results showed that normal forces are strongly changed at high loading rates, whereas adhesion forces vary slightly. The rapidly confined film behaves as a jamming liquid of enhanced viscosity for a film thickness below to a few nanometers, while the viscosity change little at slow confinement rate. These results indicate that confining rate effects play a great role in the properties of nanoconfined liquid.

Abstract: The subject was to design a fuel converter of CO₂, focusing on parameters on thermal image observation of porous media-catalyst interface during excess enthalpy reforming process. The methodology was using auto-thermal reforming technique to provide the heat of oxidation reaction. The heat provided the needed high temperature during CO₂ reduction processing. H₂-rich syngas was produced by the assistance of surface reaction of catalyst. The experiments were performed by using visualization technique for thermal image observation during reforming process. The results could be applied to the understand the temperature distribution. This study covers two parts, including thermal image observation, and reactant conversion characteristics under various reforming parameters. The experimental results show that the hybrid reformer can achieve excess enthalpy condition under the tested parameters. Additionally, the optimal CH₄ conversion efficiency can reach 93.28 %. CO₂ conversion efficiency is reaching between 0.35 and 8.65 %. The thermal image observation technique to determine catalyst local high temperature of reforming process is important in academic and practical application. Moreover, it provided information for basic research during reforming process

Abstract: Coal is currently the most widely used and most abundant fossil fuel in the world. It is primarily used for generating electricity at power plants. However, due to problems of pollution and energy consumption, importance has been placed on the development of clean coal technology. Coal-water slurry (CWS), consisting of fine coal and water mixture, is a liquid fuel used to replace heavy fuel oil for boilers and entrained flow gasifiers. Since CWS is a liquid with high viscosity and regular atomizing burners are designed for the use of fossil fuels with low viscosity, it is necessary to design high efficiency atomizing burners specific for CWS. As viscosity is a key factor for atomization characteristics, we used silicon oils of different viscosity as the testing liquids, to study the effect of different atomization parameters on the atomization characteristics. Our results show that, when the gas to liquid ratio (GLR) is high, the existing particle velocity at the central axis is lower than low GLR condition; likewise, the velocity at radial positions is higher of the high-viscosity case. The velocity also increases as the radial distance further increases away from the axis. And decrease as the GLR increases. On the other hand, the distribution of the velocities does not change after the radial distance reaches a certain limit. This limit decreases as the axial length increases. Increasing viscosity increases the inertial force of the liquid fluid, so the momentum of the atomization gas needs to be increased for it to generate enough shear stress on the fluid and to enhance the atomization characteristics.

Abstract: This paper proposes a simple and convenient numerical simulation approach to examining the characteristics and machining capacity of the manufacture process to machine a front face circular groove on a multi-tasking machine tool by a side milling cutter with special structure. In the first part of the paper, the simulation method is explained and then based on the simulation results, several important problems in applications are discussed such as how to design the cutter profile and determine the cutter position in grooving. Furthermore, an improved process is also developed for correctly machining a groove with specific profile in the required shape.

Abstract: This paper proposes a simple and convenient numerical simulation approach to examining the characteristics and machining capacity of the manufacture process to machine a front face circular groove on a multi-tasking machine tool by a side milling cutter with special structure. In the second part of the paper, the practicability of the process to machine a groove with rectangular sectional profile is discussed based on the simulation results, and then an improved process is developed for correctly machining the groove profile in the required shape. Furthermore, the effect of cutter feed motion on the profile precision of machined groove is studied; at the same time, a main disadvantage of the process as a finishing cutting method is also identified.

Abstract: In this study, the screw cutting experiments of stainless steel 15-5PH was carried out by employing the tapping center in order to investigate the tool damage of threading tap tool. Mainly, the effect of the machining conditions and the ground chamfer shape of tap tool on the thrust force and the tool wear were discussed. As a result, it was found that the thrust force was strongly influenced by the ground amount of tap tool chamfer. Especially, large change of thrust force occurred when the tap tool engaged to the pilot hole and returned out of hole.

Abstract: This paper is related to the air jet assisted machining method for a titanium alloy, Ti-6Al-4V ELI. The air jet assisted machining method is a new machining method, in which jet of the compressed air is applied to a tool tip together with flood coolant for reducing tool wear and also for extending tool life. In this experimental study, the new method was used in high-speed end milling for confirming the effect on tool life extension. Also, the optimal position of the jet nozzle was found. It was spotted that the new method is highly effective in reducing tool wear even at a high cutting speed. It is particularly noticeable that flank wear near the corner land, which is often severely damaged, was considerably reduced by the method. It turned out that the cutting forces and the degree of surface roughness observed through this method were almost the same as those through an ordinary method with flood coolant alone.